Panel display devices, such as liquid crystal display devices, require adjustment of display characteristics to obtain accurate output images for input image data. Examples of display characteristics to be adjusted include the color temperature of the white point (or the white balance) and the gamma value, for example.
The color temperature of the white point influences the color tone of the image. The specifications of many of recent display devices define that the color temperature of the white point is adjusted to 6504K (that is, the chromaticity coordinates (x, y)=(0.313, 0.329)). In this case, the setting of the display device should be adjusted so that the color temperature of the white point is kept constant for all of the allowed gray levels with respect to any grayscale images in which the gray levels of red, green and blue of every pixel are the same value.
FIG. 1 illustrates an example in which the color temperature of the white point is adjusted to 6504K in a normally-black liquid crystal display device which displays images in response to image data which represent the gray level of each color (red, green and blue) of each pixel with eight bits. With respect to all grayscale images in which the gray levels of red, green and blue are the same value, the color temperature of the white point is adjusted to 6504K for all of the allowed gray levels from 00h to FFh. It should be noted that the symbol “h” represents the hexadecimal notation.
The gamma value, on the other hand, influences the linearity or non-linearity between the gray level of image data and the actual brightness of the pixel. The input-output characteristics of a display device can be represented by the following expression:VOUT=VINγ,where VIN is the input signal or input data, VOUT is the output signal or input data, and γ is the gamma value. The specifications of many of display devices define that the gamma value γ is 2.2 and display devices are generally configured so that the gamma value is kept at 2.2, when displaying grayscale images in which the gray levels of red, green and blue of each pixel are the same value, while varying the gray levels. FIG. 2 illustrates an example in which the gamma value is adjusted to 2.2 in a normally-black liquid crystal display device which is adapted to image data which represent the gray level of each color (red, green and blue) of each pixel with eight bits (that is, image data which represent each color with 256 (=28) allowed gray levels).
One issue is that the characteristics of panel display devices, such as the color temperature of the white point and the gamma value, vary depending on the panel display devices due to the individual variations of the color filter and the backlight. In order to improve the product quality, it is desirable to compensate such individual variations.
One method for adjustment of display characteristics of a panel display device is to adjust the settings of a brightness correction circuit integrated in a display panel driver which drives the display panel. A brightness correction circuit is a sort of circuit which performs a correction calculation on input image data fed to a display panel driver, and image data obtained by the correction calculation by the brightness correction circuit (that is, corrected image data) are actually used to display images on the display panel. It is possible to adjust the color temperature of the white point and the gamma value by adjusting the settings of the brightness correction circuit and to thereby achieve compensation of the individual vitiations among display devices.
In relation to such technique, Japanese patent application publication No. H05-064037 discloses a technique in which the brightness of an optical modulated output obtained from a liquid crystal display device is measured while a video signal is externally fed to the liquid crystal display device, and the gamma correction data of a gamma correction device are adjusted on the basis of the measurement result.
The technique disclosed in this patent document, however, requires feeding the video signal (that is, image data) to the liquid crystal display device when the settings of the gamma correction device are adjusted, and this implies that it is necessary for an adjustment device used for setting the gamma correction device to have the function of feeding the video signal to the liquid crystal display device. The provision of the function of feeding the video signal to the liquid crystal display device to the adjustment device results in an increase in the cost of the adjustment device. This problem is especially severe in adjustment of settings of a brightness correction circuit of a display panel driver in a panel display device using a recent high-speed serial interface. The provision of the function of transmitting a video signal with a high-speed serial interface, which achieves a high-speed signal transmission, to the adjustment device undesirably increases the cost.